(1) Field of the Invention
The present invention relates to an improved system and method for automatically controlling a vehicle speed to a desired cruising speed applicable to vehicles.
(2) Background of the Art
A Japanese Patent Application First Publication (Tokkai) sho 60-4428 published on Jan. 10, 1985 and Japanese Utility Model Application First Publication (Zikkai) sho 61-39636 published on Mar. 13, 1986 exemplify systems for automatically controlling vehicle speeds to desired cruising speeds in which a vehicle speed is detected by means of a vehicle speed sensor, the detected vehicle speed is compared with a target vehicle speed set on the basis of a driver's decision, deviations as the result of comparison (a control deviation, first-order difference and second-order difference) are calculated, control variable is set by substituting the values of deviations into a control calculation equation, an actuator is activated on the basis of the result of a calculated equation to adjust an opening angle of an engine throttle valve, so that the vehicular engine output is adjusted to cruise the vehicle at the desired target speed.
In addition, the vehicle speed is stored into a memory at a time when the cruising speed run by the above-described system is released. Then, when the driver resumes the cruising speed run by the system, the resume operation of the system causes the vehicle to run at the stored vehicle speed.
Furthermore, such a method has been proposed to provide a more precise cruise control over the vehicle speed by professor Murakami of Kyushu Kogyo University that a fuzzy estimation is applied to the above-described system in which the control variable is set according to respective membership functions corresponding to the deviation control, first order difference, and second order difference.
However, in the above-identified Japanese Patent and Utility Model application publications, the automatic vehicle cruising speed controlling systems set the control variable only on the basis of the arithmetically operated values derived from the vehicle speed.
In addition, the input information is limited to the vehicle speed information including the control deviation, first-order difference, and second-order difference in the proposed method of fussy estimation described above.
Furthermore, since the membership function to make an input quantity or output quantity correspond to fuzzy quantity is defined, in a.,univalent correspondence, a mismatch occurs between the fuzzy quantities and control gains of the automatic cruising speed controlling system due to changes in running characteristics of the vehicle system, for example, when an automatic transmission mounted in the vehicle so as to connect to the engine output is locked-up, released or shifted down during the running on a ascending slope or descending slope.
Consequently, overshooting or undershooting to the target value of the vehicle speed occurs, control accuracy is deteriorated, and an abrupt change in the opening angle of the throttle valve occurs. Then, the comfortability of the vehicle becomes worse.
On the other hand, during the resume operation by the system to which the fuzzy estimation is applied, the same membership function is used to carry out the fuzzy estimation to set the control variable. The following problem occurs. That is to say, for example, when the vehicle speed difference is 10 Km/h as in the case where the speed ranges from 50 Km/h to 100 Km/h the change rate (acceleration) during the resume operation is largely different from the case where the difference is 50 Km/h as in a case where 50 Km/h to 100 Km/h. In the case of the small speed difference and small acceleration, a time for which the vehicle speed returns to the stored target vehicle speed is believed to be long. Whereas in the case of the large speed difference and large acceleration, the returning time is believed to be short.
In this case, the comfortability of the vehicle again becomes worsened.